The present invention relates to a plasma processing apparatus of the type used in etching or film formation; and, more particularly, the invention relates to a plasma processing apparatus that is suitable for use in the production of a semiconductor device, a liquid crystal display substrate or the like.
In correspondence with efforts directed toward increasing the miniaturization of semiconductor devices and the like, process conditions (such as the process window) for enabling the attainment of uniform processing results in wafer plasma processing have become narrower year by year, so that plasma processing apparatuses in the future will be required to achieve more perfect control of such process conditions.
In order to realize this, an apparatus is needed which can control the distribution of a plasma, the dissociation of a process gas, the surface reactions in a reactor, and the like, with an extremely high degree of precision. At present, an example of a plasma source that can be used with such an apparatus is a high frequency induction coupling type plasma source.
For example, Japanese Patent Laid-open No. 2-235332 (1990), discloses an induction coupling type plasma processing apparatus in which an induction electric field is produced in a process chamber by a high frequency coil, to thereby accelerate electrons in a plasma, and energy is given to a process gas introduced into the process chamber, whereby generation and sustention of the plasma are achieved. The high frequency coil is mounted on the exterior of the process chamber with the use of insulator, such as a quartz insulator, that constitutes part of the process chamber. The high frequency coils are generally in the shape of a loop, a coil or a spiral, and high frequency electric power with a frequency of several hundreds of kHz to several hundreds of MHz is supplied thereto, thereby producing an induction electric field.
On the other hand, there also is a high frequency induction coupling type plasma processing apparatus having a structure in which a coil is provided inside the process chamber. For example, in a plasma processing apparatus disclosed in Japanese Patent Laid-open No. 7-106095 (1995), a spiral coil, serving as a high frequency induction coil, is disposed inside the process chamber at a position opposed to a semiconductor wafer, which is the workpiece to be processed.
Here, on an electric circuit basis, these plasma processing apparatuses can each be deemed to operate as a transformer in which the plasma and the high frequency coil are inductively coupled with each other, as a result of generation of an induction current in the plasma (a transformer with the high frequency coil as a primary coil and with the current path in the plasma as a secondary coil); hence, these plasma processing apparatuses are called induction coupling type plasma processing apparatuses.
The induction coupling type plasma processing apparatuses have certain advantages, in that a plasma with a comparatively high density of 1011 to 1012 (cm−3) can be generated at a low pressure of several milliTorr, simply and inexpensively, by means of a simple coil and a high frequency power source. In addition, a plasma with a large area can be easily generated by arranging a coil in the form of a plain surface so that it is opposed to the workpiece to be processed; and, since the inside of the process chamber is simple, it may be possible to reduce the amount of foreign matter flying onto the workpiece during processing.
By use of an induction coupling type plasma processing apparatus, plasma with high density can be easily generated even at a low pressure, so that the mean free path of ions is enlarged. As a result, the directionality of ions incident on the workpiece to be processed is enhanced, so that the plasma process is suitable for minute processing, and a high processing rate can be obtained.
On the other hand, the above-mentioned apparatuses have the following problems.
For example, in the induction coupling type plasma processing apparatus disclosed in Japanese Patent Laid-open No. 2-235332 (1990) (hereinafter referred to as plasma processing apparatus 1), the high frequency coil is disposed in the atmosphere outside of the process chamber in which the plasma is generated, being mounted on the side of the process chamber through an insulator, such as quartz. Therefore, the insulator must have sufficient strength for enduring the atmospheric pressure, and when the workpiece to be processed is enlarged in area, as is expected in the future, the insulator must be enlarged in thickness accordingly. When the insulator is thus enlarged in thickness, the distance between the coil and the plasma is also enlarged. Then, as discussed, for example, in the paper by Keller et al, Journal Of Vacuum Science A11(5), September/October 1993 p.2487, the coupling condition between the coil and the plasma is worsened, the efficiency of formation of the plasma is lowered, and, as a result, the heat generation loss at the coil or a matching circuit is increased.
On the other hand, in recent years, the diameter of the wafer serving as the workpiece to be processed has progressively increased, and, as a result, it has become necessary to freely control the distribution of the plasma. In the induction coupling type plasma processing apparatus, it is possible to freely control the position of generation of plasma and to control the plasma distribution at the wafer position by, for example, varying the electric currents flowing respectively through two systems of coils and the ratio of the currents.
However, as empirically and theoretically shown in the paper by Collison et al, J. Vac. Sci. Technol. A 16-1, January/February 1998 p.100, even where two coils are used, the plasma is generated only at a mean position between the two induction coils, namely, only at one position in the vicinity of the area where the induction magnetic field of the coils is the strongest.
Thus, since the plasma is generated at one position, it is difficult to obtain a uniform plasma even by use of two systems of coils, unless the wafer is disposed at a certain distance from the coils. Therefore, in order to generate the plasma at two locations, it is necessary that the distance between the two coils is sufficiently large relative to the distance between the coils and the plasma.
However, in the case where the coil is disposed on the outside (atmosphere side) of the process chamber, as in plasma processing apparatus 1, the distance between the coil and the plasma cannot be reduced to or below a certain value. Therefore, where the distance between the two coils is enlarged, it is difficult to realize the required spacing with a substantial chamber size.
On the other hand, in the plasma processing apparatus disclosed in Japanese Patent Laid-open No. 7-106095 (1995) (hereinafter referred to as plasma processing apparatus 2), the coil is disposed in the chamber, whereby the above-mentioned problem due to coupling of the coil and the plasma can be solved, but another problem is newly generated.
First, in plasma processing apparatus 2, the high frequency coil is protected by a protective film formed of an insulator. Generally, in such an induction coupling type plasma processing apparatus, a strong plasma is generated at a position dose to the coil. Therefore, the damage to the protective film is extremely heavy, particularly in an apparatus using a reactive gas, such as a plasma etching apparatus, and so there arises the need for increased maintenance, such as frequent repair or replacement of parts.
On the other hand, since the coil itself is metallic, when the protective film thereof is broken, metal ions are generated and the semiconductor wafer is contaminated with the metal. In this case, the coil itself also needs replacement, so that much time and cost are consumed for maintenance.
In addition, in plasma processing apparatus 2, a cooling plate is disposed on the rear side of the induction coil, and this plate must be insulated from the coil. In such a structure, it is difficult to achieve thermal coupling between the cooling plate and the coil.
In particular in a vacuum, or at a low pressure, as typically provided during plasma processing, heat transfer at a contact surface of the structures becomes extremely poor. Therefore, there arises the problem in that a large cooling effect on the coil owing to the arrangement of the cooling plate cannot be expected, and sufficient cooling cannot be realized.
Further, there is another problem in that plasma with a high density, as is present on the side of the workpiece to be processed, is generated also on the rear side of the coil disposed opposite to the work. However, the plasma on the rear side of the coil is not used effectively for plasma processing of the workpiece, and, in addition, the chamber wall on the rear side is exposed to this strong plasma.
Further, in an induction coupling type plasma processing apparatus, whether the apparatus may be of the type in which the coil is disposed on the atmosphere side, as in the plasma processing apparatus 1, or of the type in which the coil is disposed on the vacuum side, as in the plasma processing apparatus 2, electrons are accelerated directly by the voltage exerted on the coil, and the plasma is generated on the basis of capacitive coupling.
In this case, in the above-mentioned plasma processing apparatuses of either type, uniformization of the potential distributed in the coil has not been taken into account, and so a question arises as to the formation of a uniform plasma.